1. Field of the Invention
The present invention relates to an EGR control apparatus for an internal combustion engine, which controls the amounts of two types of EGR gas in the engine in which the two types of EGR gas are supplied to cylinders thereof via two paths different from each other.
2. Description of the Related Art
Conventionally, as an EGR control apparatus for an internal combustion engine, one disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2004-150319 is known. This engine is a diesel engine, and is provided with a turbocharger, a low-pressure EGR device, a high-pressure EGR device, and so forth. The low-pressure EGR device recirculates part of exhaust gases from an exhaust passage on the downstream side of a turbine of the turbocharger and a catalytic device into an intake passage on the upstream side of a compressor of the turbocharger, as low-pressure EGR gas, while the high-pressure EGR device recirculates part of exhaust gases from the exhaust passage on the upstream side of the turbine of the turbocharger into the intake passage on the downstream side of the compressor of the turbocharger, as high-pressure EGR gas.
In the EGR control apparatus, the low-pressure EGR device and the high-pressure EGR device are driven according to the operating conditions of the engine, to thereby execute EGR control for controlling the respective amounts of the low-pressure EGR gas and the high-pressure EGR gas. Specifically, the EGR control is performed such that in a low-to-medium speed and low-to-medium load region, the high-pressure EGR gas is mainly recirculated, and the insufficient amount thereof is supplemented with the low-pressure EGR gas, and that in a low-to-medium speed and high-load region, only the low-pressure EGR gas is recirculated. Further, in a high-speed and low-to-high-load region, the EGR control is performed such that only the high-pressure EGR gas is recirculated.
In the case of an engine driven for lean burn operation, such as the diesel engine disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2004-150319, not only inert gas but also fresh air are contained in the EGR gas. However, when the amount of the EGR gas is controlled without taking the above fact into account, as in the EGR control apparatus disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2004-150319, the amount (or ratio) of inert gas supplied to the cylinders becomes short or excessive, which can make the combustion state of the engine unstable or result in increased exhaust emissions. Particularly when the engine is in a transient operating condition, the degree of fluctuation of the air-fuel ratio becomes large to make the amount of inert gas liable to be away from an appropriate value thereof. This makes the above-mentioned problem conspicuous.
Further, in the case of an engine provided with the low-pressure EGR device and the high-pressure EGR device, such as the engine disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2004-150319, the low-pressure EGR device is configured to have a path for recirculating exhaust gases into the intake passage longer than that of the high-pressure EGR device, so that the low-pressure EGR device has characteristics that the response thereof is lower than that of the high-pressure EGR device and dead time taken for the low-pressure EGR gas to reach the cylinders is longer than dead time taken for the high-pressure EGR gas to reach the cylinders. However, according to the above-described conventional EGR control apparatus, the EGR control is performed such that in the low-to-medium speed and low-to-medium load region, the high-pressure EGR gas is mainly recirculated, and the insufficient amount thereof is supplemented with the low-pressure EGR gas, which can make the amount of the inert gas in the cylinders short or excessive due to the above-mentioned dead time characteristic of the low-pressure EGR gas. In this case, the above-mentioned problem becomes more conspicuous.